Inlet Plenum with Shock Wave Suppression

ABSTRACT

An inlet plenum for a compressor that suppresses acoustic shock waves generated by the compressor, which inlet plenum has an inlet duct with a wall that surrounds an air flow path between a mouth of the inlet plenum and the compressor, comprises: at least one cavity that penetrates an inner surface of the wall, circumscribes the inner surface generally transversely to the air flow path and has a position along the air flow path that is generally coincident with at least one constructive interference peak of the acoustic shock waves along the inner surface; a width of the cavity along the direction of the air flow that is less than or equal to the wavelength of at least one of the acoustic shock waves; and a depth of the cavity generally perpendicular to the direction of the air flow that approximates a multiple of a quarter wavelength of at least one of the acoustic shock waves.

FIG. 1 is a front view of an inlet plenum for a compressor that incorporates a possible embodiment. FIG. 2 is a side view of an inlet plenum for a compressor that incorporates a possible embodiment. FIG. 3 is a detailed side view of a cavity in a wall of the inlet plenum shown in FIGS. 1 and 2 according to a first possible embodiment. FIG. 4 is a detailed side view of a cavity in a wall of the inlet plenum shown in FIGS. 1 and 2 according to a second possible embodiment. FIG. 5 is a detailed side view of a cavity in a wall of the inlet plenum shown in FIGS. 1 and 2 according to a third possible embodiment. FIG. 6 is a detailed side view of a cavity in a wall of the inlet plenum shown in FIGS. 1 and 2 according to a fourth possible embodiment. FIG. 7 is a detailed side view of a cavity in a wall of the inlet plenum shown in FIGS. 1 and 2 according to a fifth possible embodiment. FIG. 8 is a detailed side view of a cavity in a wall of the inlet plenum shown in FIGS. 1 and 2 according to a sixth possible embodiment. FIG. 9 is a detailed side view of a cavity in a wall of the inlet plenum shown in FIGS. 1 and 2 according to a seventh possible embodiment. FIG. 10 is a detailed side view of a cavity in a wall of the inlet plenum shown in FIGS. 1 and 2 according to an eighth possible embodiment. FIG. 11 is a detailed side view of a cavity with a flow deflector in a wall of the inlet plenum shown in FIGS. 1 and 2 according to a ninth embodiment.

FIGS. 1 and 2 are front and side views, respectively, of an inlet plenum 2 for a compressor 4 that incorporates a first possible embodiment. FIG. 3 is a detailed side view of a cavity 6 in a wall 8 of the inlet plenum 2 according to the first possible embodiment. Referring to FIGS. 1 through 3 together, the wall 8 of the inlet plenum 2 may have one or more cavities 6, such as the two cavities 6 shown in FIGS. 1 and 2. Each cavity 6 penetrates an inner surface 10 of the wall 8 and circumscribes the inner surface 10 generally transversely to an air flow path 12 between a mouth 14 of the inlet plenum 2 and the compressor 4.

The compressor 4 generates acoustic shock waves that propagate from the compressor 4 opposite the direction of the air flow path 12 out of the mouth 14 of the inlet plenum 2 into the ambient air. The content of such acoustic shock waves comprises a combination of broadband noise and compressor blade passing frequencies. As the acoustic shock waves propagate toward the mouth 14, they generate constructive interference peaks 16 at various points along the inner surface 10 of the wall 8. Attenuation of these constructive interference peaks 16 result in a far lower level of noise due to the acoustic shock waves that emanate from the mouth 14 of the inlet plenum 8.

The positions of the cavities 6 are generally coincident with at least some of the constructive interference peaks 16. It is possible to determine the positions of the constructive interference peaks 16 by empirical measurement or acoustic optimising software, such as the acoustic optimising software known as “COMSOL”. The cavities 6 act as acoustic traps by means of reactive phase cancellation of at least the predominant wavelength of the acoustic shock waves at their respective constructive interference peaks 16.

To secure such reactive phase cancellation, the width 18 of each cavity 6 between side surfaces 20 of the cavity 6 along the direction of the air flow 12 is less than or equal to the predominant wavelength of the respective constructive interference peak 16 to form an effective band pass filter, and the depth 22 of each cavity 6 between the inner surface 10 and a bottom surface 24 of the cavity 6 generally perpendicular to the direction of the air flow 12 approximates a multiple of the predominant wavelength of the respective constructive interference peak 16 to achieve reactive phase cancellation at such wavelength. In FIG. 3, the width 18 is approximately one half wavelength and the depth 22 is approximately one quarter wavelength. For instance, if the compressor 4 generates a blade passing frequency of approximately 12,000 Hz, and this frequency corresponds to the predominant wavelength of the respective constructive interference peak 16, which at sea level and an ambient air temperature of approximately 68 degrees Fahrenheit the speed of sound is approximately 1126 feet per second and the wavelength would be approximately 1 and ⅛ inch, the width 18 would be approximately 9/16 inch and the depth 22 would be approximately 9/32 inch.

It is possible for the constructive interference peaks 16 to correspond to different wavelengths as the acoustic shock waves propagate from the compressor 4 toward the mouth 14 of the inlet plenum 2. Therefore, each cavity 6 along the wall 8 may have a different width 18 and depth 22 to correspond to a different predominant wavelength of the acoustic shock waves.

It is possible to form the cavities 6 in the wall 8 in different ways. FIG. 3 shows the formation of the cavity 6 by means of corrugation of the wall 8. Such corrugation may be achievable by various means, such as by stamping or moulding. FIG. 4 is a detailed side view of a second possible embodiment that is similar to the first embodiment shown in FIG. 3, but the formation of the cavity 6 is by means of indentation of the inner surface 10 of the wall 8. Such indentation may be achievable by various means, such as moulding, machining, embossing or incising the inner surface 10 of the wall 8.

The side surfaces 20 of each cavity 6 may be generally parallel to each other, as shown in FIGS. 3 and 4. Alternatively, to achieve acoustic impedance matching or to broaden attenuation band pass, the side surfaces 20 of each cavity 6 may be non-parallel to each other. FIG. 5 is a detailed side view of a third possible embodiment that is similar to the first embodiment shown in FIG. 3, but one of the side surfaces 20 of the cavity 6 has a cant to be non-parallel to the other side surface 20. Similarly, FIG. 6 is a detailed side view of a fourth possible embodiment that is also similar to the first embodiment shown in FIG. 3, but wherein both side surfaces 20 have a cant and non-parallel to each other.

Acoustic damping material may at least partially fill each cavity 6 for the purpose of broadening its attenuation band pass or increasing its effective wavelength. FIG. 7 is a detailed side view of a fifth possible embodiment that is similar to the first embodiment shown in FIG. 3, but an acoustic damping material 26 fills the cavity 6.

FIG. 8 is a detailed side view of a fifth possible embodiment wherein the depth 22 of the cavity 6 is a full wavelength of the respective constructive interference peak 16. Furthermore, the side surfaces 20 of the cavity 6 both have cants to be nonparallel to each other. The width 18 of the cavity 6 is approximately one half wavelength of the respective constructive interference peak 16 at the bottom side 24 of the cavity 6.

FIG. 9 is a detailed side view of a sixth possible embodiment wherein both the width 18 and the depth 22 of the cavity 6 is approximately a full wavelength of the respective constructive interference peak 16. FIG. 10 is a detailed side view of a seventh possible embodiment that is similar to the sixth possible embodiment shown in FIG. 9, except that the acoustic damping material 26 fills a portion of the cavity 6. FIG. 10 shows a depth 28 of the acoustic damping material 26 that approximates one quarter wavelength of the respective constructive interference peak 16. FIG. 11 is a detailed side view of an eight possible embodiment that is similar to the third possible embodiment wherein the width 18 of the cavity 6 is approximately a full wavelength and the depth of the cavity 6 is approximately one quarter wavelength of the respective constructive interference peak 16. An air flow deflector 30 extends along a portion of the width 18 of the cavity 6 from the upstream side surface 20 of the cavity 6 along the inner surface 10 of the wall 8 that partially covers the cavity 6. The air flow deflector minimises any turbulence from air that travels through the air flow path 12.

The described embodiments as set forth herein represents only some illustrative implementations of the invention as set forth in the attached claims. Changes and substitutions of various details and arrangement thereof are within the scope of the claimed invention. 

1. An inlet plenum for a compressor that suppresses acoustic shock waves generated by the compressor, which inlet plenum has an inlet duct with a wall that surrounds an air flow path between a mouth of the inlet plenum and the compressor, comprising: at least one cavity that penetrates an inner surface of the wall, circumscribes the inner surface generally transversely to the air flow path and has a position along the air flow path that is generally coincident with at least one constructive interference peak of the acoustic shock waves along the inner surface; a width of the cavity along the direction of the air flow path that is less than or equal to the wavelength of at least one of the acoustic shock waves; and a depth of the cavity generally perpendicular to the direction of the air flow path that approximates a multiple of a quarter wavelength of at least one of the acoustic shock waves.
 2. The inlet plenum of claim 1, wherein corrugation of the wall forms the cavity.
 3. The inlet plenum of claim 1, wherein indentation of the inner surface forms the cavity.
 4. The inlet plenum of claim 1, wherein multiple cavities along the inner surface have positions generally coincident with respective multiple constructive interference peaks of the acoustic shock waves along the inner surface.
 5. The inlet plenum of claim 4, wherein some of the multiple constructive interference peaks correspond to acoustic shock waves of different wavelengths and each one of the multiple cavities has its depth corresponding to a multiple of a quarter of the wavelength of its corresponding constructive interference peak.
 6. The inlet plenum of claim 1, further comprising acoustic damping material that at least partially fills the cavity.
 7. The inlet plenum of claim 1, wherein the width of the cavity is equal to one wavelength of at least one of the acoustic shock waves.
 8. The inlet plenum of claim 1, wherein side surfaces of the cavity are generally parallel to each other.
 9. The inlet plenum of claim 1, wherein side surfaces of the cavity are generally non-parallel to each other.
 10. The inlet plenum of claim 1, wherein the depth of the cavity is equal to one quarter wavelength of at least one of the acoustic shock waves.
 11. The inlet plenum of claim 10, wherein an acoustic dampening material fills the cavity.
 12. The inlet plenum of claim 1, further comprising an air flow deflector that extends along a portion of the width of the cavity from an upstream side of the cavity along the inner surface of the wall.
 13. An inlet plenum for a compressor that suppresses acoustic shock waves generated by the compressor, which inlet plenum has an inlet duct with a wall that surrounds an air flow path between a mouth of the inlet plenum and the compressor, comprising: multiple cavities that penetrate an inner surface of the wall, circumscribe the inner surface generally transversely to the air flow path and each have a position along the air flow path that is generally coincident with a corresponding constructive interference peak of the acoustic shock waves along the inner surface; a width of each cavity along the direction of the air flow that is less than or equal to the wavelength of at least one of the acoustic shock waves at its corresponding constructive interference peak; and a depth of each cavity generally perpendicular to the direction of the air flow that approximates a multiple of a quarter wavelength of at least one of the acoustic shock waves at its corresponding constructive interference peak.
 14. The inlet plenum of claim 13, wherein corrugation of the wall forms each cavity.
 15. The inlet plenum of claim 13, wherein indentation of the inner surface forms each cavity.
 16. The inlet plenum of claim 13, further comprising acoustic damping material that at least partially fills the cavity.
 17. The inlet plenum of claim 13, wherein the width of the cavity is equal to one wavelength of at least one of the acoustic shock waves.
 18. The inlet plenum of claim 13, wherein the depth of the cavity is equal to one quarter wavelength of at least one of the acoustic shock waves.
 19. The inlet plenum of claim 13, further comprising an air flow deflector that extends along a portion of the width of each cavity from an upstream side of the cavity along the inner surface of the wall.
 20. An inlet plenum for a compressor that suppresses acoustic shock waves generated by the compressor, which inlet plenum has an inlet duct with a wall that surrounds an air flow path between a mouth of the inlet plenum and the compressor, comprising: multiple cavities that penetrate an inner surface of the wall, circumscribe the inner surface generally transversely to the air flow path and each have a position along the air flow path that is generally coincident with a corresponding constructive interference peak of the acoustic shock waves along the inner surface; a width of each cavity along the direction of the air flow path that is less than or equal to the wavelength of at least one of the acoustic shock waves at its corresponding constructive interference peak; and a depth of each cavity generally perpendicular to the direction of the air flow path that approximates a quarter wavelength of at least one of the acoustic shock waves at its corresponding constructive interference peak.
 21. The inlet plenum of claim 20, further comprising an air flow deflector that extends along a portion of the width of each cavity from an upstream side of the cavity along the inner surface of the wall. 